The present invention relates to pilot operated hydraulic valves which are pressure balanced, and particularly to such valves that are bidirectional.
Construction and agricultural equipment have moveable members which are operated by an actuator, such as a hydraulic cylinder and piston arrangement that is controlled by a hydraulic valve. There is a present trend with respect to construction and agricultural equipment away from manually operated hydraulic valves toward electrical controls and the use of solenoid valves. This type of control simplifies the hydraulic plumbing as the control valves do not have to be located in the operator cab. This change in technology also facilitates computerized control of various machine functions.
Application of pressurized hydraulic fluid from a pump to the actuator can be controlled by a set of proportional solenoid valves of a type described in U.S. Pat. No. 5,878,647. When an operator desires to move a member on the equipment a control lever is operated to send signals to the solenoid valves for the cylinder associated with that member. One solenoid valve is opened to supply pressurized fluid to the cylinder chamber one side of the piston and another solenoid valve opens to allow fluid being forced from the opposite cylinder chamber to drain to a reservoir, or tank. By varying the degree to which the solenoid valves are opened, the rate of flow into the associated cylinder chamber can be varied, thereby moving the piston at proportionally different speeds.
Solenoid operated pilot valves are well known for controlling the flow of hydraulic fluid and employ an electromagnetic coil which moves an armature in one direction to open a valve. The armature acts on a pilot poppet that controls the flow of fluid through a pilot passage in a main valve poppet. The amount that the valve opens is directly related to the magnitude of electric current applied to the electromagnetic coil, thereby enabling proportional control of the hydraulic fluid flow. Either the armature or another valve member is spring loaded to close the valve when electric current is removed from the solenoid coil.
A drawback of conventional solenoid operated pilot valves results from effects produced by the pressure differential that develops across the valve in the closed state. That pressure differential changes with variation in the load applied to the equipment component that is operated by the hydraulic fluid from the valve. The load and the supply pressure variation affect the pressure at the valve's outlet and produces that pressure differential across the valve. In the closed state, the pressure differential can affect the amount of force required to open the valve and required to produce a given flow rate of the hydraulic fluid. Therefore in a solenoid operated valve, variation in this pressure differential affects the magnitude of electrical current required to operate the valve.
This problem was addressed by providing a pressure balancing stem in the pilot valve passage, as shown in U.S. Pat. No. 5,878,647. This balanced the pressures so that the only forces acting on the pilot poppet were those from the return spring and the electromagnetic coil. While this overcame the effects due to a pressure imbalance in other designs, fluid leakage often occurred along the balancing stem. Attempts to provide a better seal against that leakage increased friction acting on the stem and created a sticking problem that adversely affected smooth valve operation.
Conventional pilot-operated poppet valves are single directional. There is an inlet port and an outlet port of the valve and the pressure at the inlet port is communicated to the pilot control chamber thus enabling the valve to open when the inlet port pressure is greater than the pressure at the outlet port. This enables fluid to flow from the inlet port to the outlet port. Because of this arrangement, the valve can not be used to control the flow of fluid in the reverse direction from the outlet port to the inlet port. In some hydraulic systems, a bidirectional flow is desired to be controlled. To accommodate flow in both direction a second valve connected in a reverse parallel manner to the first valve was required. Therefore, it is desirable to create bidirectional pilot-operated poppet valve.